Stereoscopic three dimensional (3d) projection systems have been used for many years. One technology known to the art and described, for example, in U.S. Pat. No. 7,528,906 B2 issued May 5, 2009 and entitled “Achromatic Polarization Switches”, describes how a polarization modulator can be placed in-front of a single-lens projector, such as a 3-chip DLP digital cinema projector or otherwise.
The projector is arranged so as to generate a single image-beam comprising a rapid succession of alternate left and right-eye images at high speeds of typically 144 Hz (hertz). The polarization modulator then imparts an optical polarization state to images generated by said projector and said polarization modulator is typically operated in synchronization with said projector in order to arrange for all left-eye images to possess a first state of circular polarization and all right-eye images to possess a second state of circular polarization, with said first and second states of circular polarization being mutually orthogonal (i.e possessing opposite senses of rotation, for example with said first circular polarization state comprising clockwise or right-handed circular polarization and said second circular polarization state comprising anti-clockwise or left-handed circular polarization).
Thereafter, said left and right-eye images are focused onto the surface of a polarization-preserving projection-screen such as a silver-screen or otherwise, thereby enabling the viewing of time-multiplexed stereoscopic 3d images via utilization of passive circular-polarized viewing-glasses. It will be known to one skilled-in-the-art that the utilization of passive circular-polarized viewing-glasses enables the observer to tilt their head without there being a significant reduction in the optical performance of said stereoscopic 3d projection system thereof. It is therefore for this reason the majority of passive stereoscopic 3d projection systems currently on the market are based on circular polarization according to the prior-art.
Furthermore, it will be known to one skilled-in-the-art that said polarization modulator may comprise of at least one or more liquid crystal elements stacked together in order to achieve the required electro-optical switching characteristics. One technology known to the art for fulfilling this requirement and described, for example, in U.S. Pat. No. 7,760,157 B2 issued Jul. 20, 2010 and entitled “Enhanced ZScreen modulator techniques”, describes how said polarization modulator may comprise of two individual pi-cell liquid crystal elements stacked together in mutually crossed orientation such that the surface alignment directors in the first pi-cell are orthogonal to the surface alignment directors in the second pi-cell thereof. Pi-cell liquid crystal elements are known to the art and characterized by their surface alignment directors on both substrates being aligned mutually parallel. Therefore, in at least one optical state the liquid crystal materials composing said pi-cell form a helical structure between said substrates with an overall twist of 180 degrees (i.e pi or π radians). A detailed description of the design and function of pi-cell liquid crystal elements can be found elsewhere in the literature according to the prior-art.
Moreover, each pi-cell liquid crystal element can, for example, be rapidly switched between a first optical state possessing an optical retardation value that is substantially equal to zero when driven with high voltage (eg. 25 volt) in order to switch said liquid crystal materials to the homeotropic texture, and a second optical state possessing an optical retardation value that is substantially equal to 140 nm (nanometers) when driven with a low voltage (eg. 3 volt) in order to switch said liquid crystal materials to the splay texture. The homeotropic texture is characterized by the molecular axes of said liquid crystal materials being aligned substantially perpendicular to the surfaces of both substrates, whereas the splay texture is characterized by said molecular axes for said liquid crystal materials being aligned substantially parallel with the surfaces of said substrates and additionally with the twist within said liquid crystal materials being substantially equal to zero. Furthermore, said pi-cell liquid crystal elements are capable of being rapidly switched between said first and second optical states thereof at speeds of greater than typically 250 μs (microseconds) and are therefore often used when designing such polarization modulators according to the state-of-the-art.
It will also be known to one skilled-in-the-art that when one of said pi-cell liquid crystal elements is in said second optical state possessing a retardation value substantially equal to 140 nm, then said pi-cell liquid crystal element constitutes an optical Quarter-Wave-Plate (QWP) for the central part of the visible wavelength spectrum and will therefore convert incident linearly polarized visible light to circular polarization.
Therefore, by stacking together two individual pi-cell liquid crystal elements in mutually crossed orientation together with a linear polarization-filter located at the input surface of said stack of liquid crystal elements in order to first convert the initially randomly polarized (i.e unpolarized) incident light generated by said projector to linear polarization, then the images generated by said projector can be rapidly modulated between left and right circular polarization states by operating said pi-cell liquid crystal elements mutually out-of-phase according to the state-of-the-art. Specifically, when said first pi-cell liquid crystal element is operated with high voltage (i.e liquid crystal materials are switched to said homeotropic texture) then said second pi-cell liquid crystal element is simultaneously operated with low voltage (i.e liquid crystal materials are switched to said splay texture), and vice versa according to the state-of-the-art.
However, it will be understood by one skilled-in-the-art that in order for each of said pi-cell liquid crystal elements to generate circular polarization from visible light that is initially linearly polarized, the product of the anisotropic index of refraction (Δn) and cell-gap (d) for each of said pi-cell liquid crystal elements thereof is mandated to be substantially equal to 0.14 μm (micrometers), corresponding to a QWP for the central part of the visible wavelength spectrum.
This therefore limits the minimum permissible value for the product of the anisotropic index of refraction (Δn) and cell-gap (d) for said liquid crystal elements thereof when utilizing circular polarization according to the state-of-the-art. Moreover, since the switching speed of said liquid crystal elements is directly related to said product of anisotropic index of refraction (Δn) and cell-gap (d), this therefore limits the maximum speed at which said liquid crystal elements are able to switch between said first and second optical states thereof. Specifically, it will be known to one skilled-in-the-art that the switching speed of said liquid crystal elements is directly related to both (i) the reciprocal of the cell-gap (d), and (ii) the viscosity of the liquid crystal materials. Moreover, the viscosity of said liquid crystal materials principally depends upon the value of the dielectric anisotropy (Δε), which in turn is related to the value of the anisotropic index of refraction (Δn).
In practice, the maximum switching speed of pi-cell liquid crystal elements designed to generate circular polarization when operating together with visible light is typically only approximately 250 μs, and this relatively slow speed therefore limits both the maximum frame-rate at which said pi-cell liquid crystal elements are able to operate, as well as limiting the maximum optical light efficiency that can be achieved due to the necessity of utilizing a relatively long dark-time period for the projector. The dark-time period is defined as being the time interval between successive images generated by said projector, and during said dark-time period no light is emitted by said projector.
Furthermore, since the images generated by a typical 3-chip DLP digital cinema projector are initially randomly polarized (i.e unpolarized), then it will be known to one skilled-in-the-art that the linear polarization-filter located at the input surface of said polarization modulator will absorb approximately 50% of the incoming light initially generated by said projector. This will therefore also significantly reduce the overall optical light efficiency of the aforementioned system, thereby resulting in the creation of time-multiplexed stereoscopic 3d images according to the state-of-the-art that are severely lacking in on-screen image brightness.
One technology known to the art for increasing the overall optical light efficiency of a stereoscopic 3d projection system and described, for example, in U.S. Pat. No. 7,857,455 B2 issued Dec. 28, 2010 and entitled “Combining P and S rays for bright stereoscopic projection”, and again in U.S. Pat. No. 8,220,934 B2 issued Jul. 17, 2012, and entitled “Polarization conversion systems for stereoscopic projection”, uses a polarization beam-splitting element in order to split the incoming randomly polarized incident image-beam generated by a single-lens projector into one primary image-beam propagating substantially in the same direction as said original incident image-beam and possessing a first state of linear polarization (eg. P type linear polarization), and one secondary image-beam propagating substantially in a perpendicular direction relative to said incident image-beam and possessing a second state of linear polarization (eg. S type linear polarization), with said first and second states of linear polarization being mutually orthogonal.
Thereafter, a reflecting surface such as a mirror or otherwise is used to deflect the optical path of said secondary image-beam towards the surface of a polarization-preserving projection-screen, thereby enabling both said primary and secondary image-beams to be arranged so as to mutually overlap to a substantial extent on the surface of said projection-screen thereof. The aforementioned double image-beam system according to the state-of-the-art therefore enables both the S and P linear polarization components composing said initial incident image-beam generated by said projector to be used in order to recreate the overall on-screen image, thereby increasing the resulting image brightness.
Additionally, a polarization rotator is typically required in order to rotate the linear polarization state of said secondary image-beam by substantially 90 degrees and ensure that both said primary and secondary image-beams thereafter possess the same linear state of polarization. Furthermore, one or more polarization modulators are located within the optical paths for each of said primary and secondary image-beams thereof and operated in synchronization with said projector in order to arrange for all left-eye images to possess a first state of circular polarization and all right-eye images to possess a second state of circular polarization, with said first and second states of circular polarization being mutually orthogonal.
However, it will once again be understood by one skilled-in-the-art that in order for said pi-cell liquid crystal elements composing at least one of said polarization modulators to convert said linearly polarized visible incident light to circular polarization, the product of the anisotropic index of refraction (Δn) and cell-gap (d) for each of said liquid crystal elements thereof is mandated to be substantially equal to 0.14 μm, corresponding to a QWP for the central part of the visible spectrum. This therefore once again limits the minimum permissible value for said product of anisotropic index of refraction (Δn) and cell-gap (d) for said liquid crystal elements thereof, thereby also limiting the maximum switching speed that can be achieved by said liquid crystal elements according to the state-of-the-art. This results in there being a limitation on both the maximum frame-rate as well as the maximum optical light efficiency that can be achieved by said stereoscopic 3d projection system that is based on circular polarization according to the prior-art.
The double image-beam system described above according to the state-of-the-art also has the disadvantage in that there is a relatively large optical path-length difference between said primary and secondary image-beams thereof, thereby typically requiring the use of a telephoto-lens pair and/or the deformation of said reflecting-surface in order to compensate for said optical path-length difference. However, this adds both complexity and expense to the overall system.
An improved system for the displaying of high brightness time-multiplexed stereoscopic 3d images according to the state-of-the-art is described, for example, in U.S. Patent Application Publication No. 2015/0103318 A1 dated Apr. 16, 2015 and entitled “Stereoscopic image apparatus”, and again in U.S. Pat. No. 9,740,017 B2 issued Aug. 22, 2017 and entitled “Optical polarization device for a stereoscopic image projector”. Here, a beam-splitting element is used to separate the randomly polarized incident image-beam generated by a single-lens projector into one primary image-beam propagating substantially in the same direction as said original incident image-beam and possessing a first state of linear polarization (eg. P type linear polarization), and two secondary image-beams propagating in mutually opposite directions that are also both substantially perpendicular to said original incident image-beam thereto and possessing a second state of linear polarization (eg. S type linear polarization), with said first and second linear polarization states being mutually orthogonal.
Thereafter, reflecting surfaces such as mirrors or otherwise are used to deflect the optical paths for each of said secondary image-beams towards a polarization-preserving projection-screen and arranged such that said primary and secondary image-beams partially overlap in order to mutually combine and recreate a complete image on the surface of said projection-screen thereto. The aforementioned triple image-beam system therefore enables both the S and P linear polarization components composing said original incident image-beam generated by said projector to be used in order to recreate the overall on-screen image, thereby ensuring for a higher level of image brightness as compared to a single image-beam system.
Additionally, polarization modulators are located within the optical paths for each of said primary and secondary image-beams thereof and operated so as to modulate the polarization states of said image-beams in synchronization with the images generated by said projector. Moreover, said polarization modulators typically each comprise a stack of two individual pi-cell liquid crystal elements aligned in mutually crossed orientation and arranged so as to convert the linear polarization states of said primary and secondary image-beams to circular polarization. Specifically, it is arranged such that all left-eye images possess a first state of circular polarization, and all right-eye images possess a second state of circular polarization, with said first and second states of circular polarization being mutually orthogonal. A time-multiplexed stereoscopic 3d image can hence be viewed on the surface of said projection-screen via utilization of passive circular-polarized viewing-glasses.
It will be understood by one skilled-in-the-art that the aforementioned triple image-beam system according to the state-of-the-art possesses a relatively small optical path-length difference between said primary and secondary image-beams as compared to the previously described double image-beam system thereto, thereby eliminating the necessity of utilizing a telephoto-lens pair in order to compensate for said optical path-length difference. This therefore reduces the overall complexity and cost of said triple image-beam system according to the prior-art.
However, once again said triple image-beam system according to the state-of-the-art utilizes circular polarization in order to enable the observer to tilt their head without there being a significant reduction in the overall optical performance. It will therefore be understood by one skilled-in-the-art that this criterion mandates the product of the anisotropic index of refraction (Δn) and cell-gap (d) for each of said liquid crystal elements thereof to be substantially equal to 0.14 μm, thereby limiting the maximum switching speed at which said liquid crystal elements can operate. This in turn limits both the maximum frame-rate and the maximum optical light efficiency that can be achieved by said liquid crystal elements thereof, hence limiting the overall performance of said time-multiplexed stereoscopic 3d projection system according to the state-of-the-art.